The present invention relates to a weft sensing system for a weaving machine, and more specifically to a weft sensing system of an optical type.
Japanese Utility Model Provisional Publication No. 62-11192 shows an air jet loom equipped with an optical sensor unit including a prism, a light emitting device and a light sensitive device. The sensor unit is inserted in an interspace between two adjacent reed dents from the back side, and designed so that light is intercepted by a weft yarn when the weft yarn is correctly inserted in a warp shed, and reaches a preset goal of a weft flight path. This sensor unit, however, requires troublesome removal, reinstallment, and readjustment in case of warp exchange because the sensor unit is attached to the reed. Furthermore, this sensing unit must cover a relatively wide portion of the weft flight path to correctly detect the weft yarn. If the light emitting and sensing devices are placed on the front side of the reed, then the sensing accuracy becomes worse because the weft yarn occupies a very small portion in a relatively large sensing area.
Japanese Patent Provisional Publication No. 63-275755 shows an air jet loom equipped with a weft sensing system which can detect a broken pick. This sensing system, however, employs two optical sensing units which are attached to the reed at two separate positions. Therefore, this sensing system is complicated. Moreover, the positions of the two sensing units must be adjusted separately according to the kinds of thread and fabric.